Radio-controlled-clock (RCC) devices that rely on time signal broadcasts have become widely used in recent years. A radio-controlled-clock (RCC) is a timekeeping device that provides the user with accurate timing information that is derived from a received signal, which is broadcast from a central location, to allow multiple users to be aligned or synchronized in time. Colloquially, these are often referred to as “atomic clocks” due to the nature of the source used to derive the timing at the broadcasting side. In the United States, the National Institute of Standards and Technology (NIST) provides such broadcast in the form of a low-frequency (60 kHz) digitally-modulated signal that is transmitted at high power from radio station WWVB in Fort Collins, Colo. The information encoded in this broadcast includes the official time of the United States.
Similar services operating at low frequencies exist in other regions worldwide, including Germany and Japan. Many consumer-market products exist, including watches, alarm clocks and wall clocks, that are capable of receiving one or more of these broadcasts and which can display the correct time to within approximately one or two seconds of accuracy. While the broadcast may be active continuously, a typical radio-controlled clock may be set to receive the broadcast only once a day. Such reception, if successful (depending on the condition of the wireless link and potential interference), is typically used to reset the timekeeping device, such that if it were set incorrectly or has drifted away from the correct time, it will be set in accordance to the time communicated through the broadcast signal.
Reception of the time signal, however, is being challenged by a growing number of sources of electromagnetic interference. In particular, the on-frequency interference from the MSF radio station in the United Kingdom has been identified as a particularly challenging jammer for receivers on the East Coast.
The frequency of operation of WWVB is 60 kHz, for which the receiving antenna is typically an inductor based on a ferrite rod. The ferrite rod antenna structure, being based on multiple loops wrapped around the ferrite core, offers maximal signal pick-up when the rod is placed in the magnetic field, such that flux flows through the rod, allowing current to be induced into the loops in accordance with Faraday's law of induction.
When the rod is tilted (i.e. placed off axis) with respect to the direction of the magnetic flux, however, a loss whose magnitude corresponds to cos θ is experienced, where θ represents the angle by which the rod deviates from the optimal direction. The optimal orientation is when the rod is oriented broadside with respect to the transmitter, since a vertical radiating element is used for the broadcast antenna in Fort Collins, which results in vertical polarization.
It is noted that when the antenna's orientation is perpendicular to the optimal orientation, a null is experienced for which, theoretically, the magnitude of the signal induced in the antenna would be zero. This results in a total loss of reception.
Many users of radio controlled clocks are not aware of the antenna in their products, and/or not aware of the antenna's orientation and its significance, and consequently do not orient them in the optimal direction. Thus, significant losses might be experienced that could result in loss of reception.
Statistical analysis, based on the known probability-density-function (PDF) of the cosine function, shows that these losses may exceed 15 dB with a probability of 11%, which means that one out of nine users experiences losses higher than 15 dB, when assuming uniformly distributed angles of orientation amongst the many users.
There is thus a need for an improved antenna scheme for reception of time signal broadcasts such as that provided by WWVB in the United States and radio stations in other countries that cost-effectively addresses the above outlined reception challenges.